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                <h1 id="连接查询"><a href="#连接查询" class="headerlink" title="连接查询"></a>连接查询</h1><p>又称多表查询，多表连接，当查询的字段来自多个表时，就会用到多表查询</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> name<span class="token punctuation">,</span>boyName <span class="token keyword">from</span> beauty<span class="token punctuation">,</span>boys<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>笛卡尔乘积现象</p>
<p>表1有m行、表2有n行，结果为m*n行</p>
<p>即beauty中的每一项都对应一次boys中的所有项</p>
<p>发生原因：没有有效的连接条件</p>
<p>如何避免：添加有效的连接条件</p>
<p>分类：</p>
<p>​            按年代分类：</p>
<p>​            sql192标准：仅仅支持内联</p>
<p>​            sq199标准【推荐】：支持内连接+外连接（左外和右外）+交叉连接</p>
<p>​            按功能分类：</p>
<p>​            内连接：</p>
<p>​                        等值连接</p>
<p>​                        非等值连接</p>
<p>​                        自连接</p>
<p>​            外连接</p>
<p>​                        左外连接</p>
<p>​                        右外连接</p>
<p>​                        全外连接</p>
<p>​            交叉连接</p>
<h2 id="一、sql192标准"><a href="#一、sql192标准" class="headerlink" title="一、sql192标准"></a>一、sql192标准</h2><h3 id="1、等值连接"><a href="#1、等值连接" class="headerlink" title="1、等值连接"></a>1、等值连接</h3><p>1）多表等值连接结果为多表的交集部分</p>
<p>2）n表连接，至少需要n-1个连接条件</p>
<p>3）多表的顺序没有要求</p>
<p>4）一般需要为表起别名</p>
<p>5）可以搭配前面介绍的所有子句使用，比如排序，分组、筛选</p>
<p>案例1：查询女神名对应的男神名</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> name<span class="token punctuation">,</span>boyName <span class="token keyword">from</span> boys<span class="token punctuation">,</span>beauty <span class="token keyword">where</span> beauty<span class="token punctuation">.</span>boyfriend <span class="token operator">=</span> boys<span class="token punctuation">.</span>id<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>案例2：查询员工名、工种号、工种名</p>
<p>起别名：</p>
<p>提高简洁度</p>
<p>区分多个重名字段</p>
<p>注意：如果为表起了别名，则查询的字段就不能使用原来的表名去限定</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> last_name<span class="token punctuation">,</span>e<span class="token punctuation">.</span>job_id<span class="token punctuation">,</span>job_title
<span class="token keyword">from</span> employees e<span class="token punctuation">,</span>jobs j
<span class="token keyword">where</span> employees<span class="token punctuation">.</span>job_id <span class="token operator">=</span> jobs<span class="token punctuation">.</span>job_id<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<p>两个表的顺序可以调换</p>
<p>可以加筛选</p>
<p>案例：查询有奖金的员工名、部门名</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> last_name<span class="token punctuation">,</span>department_name
<span class="token keyword">from</span> employees e<span class="token punctuation">,</span>departments d
<span class="token keyword">where</span> e<span class="token punctuation">.</span>department_id <span class="token operator">=</span> d<span class="token punctuation">.</span>department_id
<span class="token operator">and</span> e<span class="token punctuation">.</span>commission_pct <span class="token operator">is</span> <span class="token operator">not</span> <span class="token boolean">null</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>可以加分组</p>
<p>查询每个城市的部门个数</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> <span class="token function">count</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span> 个数<span class="token punctuation">,</span>city
<span class="token keyword">from</span> departments d<span class="token punctuation">,</span>locations l
<span class="token keyword">GROUP</span> <span class="token keyword">BY</span> city<span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<p>查询有奖金的每个部门的部门名和部门的领导编号和该部门的最低工资</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> d<span class="token punctuation">.</span>department_name<span class="token punctuation">,</span>d<span class="token punctuation">.</span>manager_id<span class="token punctuation">,</span><span class="token function">min</span><span class="token punctuation">(</span>e<span class="token punctuation">.</span>salary<span class="token punctuation">)</span>
<span class="token keyword">from</span> employees e<span class="token punctuation">,</span>departments d
<span class="token keyword">WHERE</span> e<span class="token punctuation">.</span>department_id<span class="token operator">=</span>d<span class="token punctuation">.</span>department_id <span class="token operator">and</span> e<span class="token punctuation">.</span>commission_pct <span class="token operator">is</span> <span class="token operator">not</span> <span class="token boolean">null</span>
<span class="token keyword">GROUP</span> <span class="token keyword">BY</span> d<span class="token punctuation">.</span>department_name<span class="token punctuation">,</span>d<span class="token punctuation">.</span>manager_id<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>可以加排序</p>
<p>查询每个工种的工种名和员工个数，并且按员工个数降序</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">SELECT</span> j<span class="token punctuation">.</span>job_title<span class="token punctuation">,</span><span class="token function">count</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span>
<span class="token keyword">from</span> employees e<span class="token punctuation">,</span>jobs j
<span class="token keyword">where</span> e<span class="token punctuation">.</span>job_id <span class="token operator">=</span> j<span class="token punctuation">.</span>job_id
<span class="token keyword">GROUP</span> <span class="token keyword">BY</span> e<span class="token punctuation">.</span>job_id
<span class="token keyword">ORDER</span> <span class="token keyword">BY</span> <span class="token function">count</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span> <span class="token keyword">desc</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>



<p>可以实现三表连接</p>
<p>查询员工名、部门名和所在的城市</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> e<span class="token punctuation">.</span>last_name<span class="token punctuation">,</span>d<span class="token punctuation">.</span>department_name<span class="token punctuation">,</span>l<span class="token punctuation">.</span>city
<span class="token keyword">from</span> employees e<span class="token punctuation">,</span>departments d<span class="token punctuation">,</span>locations l
<span class="token keyword">where</span> e<span class="token punctuation">.</span>department_id <span class="token operator">=</span> d<span class="token punctuation">.</span>department_id <span class="token operator">and</span>
d<span class="token punctuation">.</span>location_id <span class="token operator">=</span> l<span class="token punctuation">.</span>location_id
<span class="token operator">and</span> city <span class="token operator">like</span> <span class="token string">'s%'</span>
<span class="token keyword">ORDER</span> <span class="token keyword">BY</span> d<span class="token punctuation">.</span>department_name <span class="token keyword">desc</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>



<h3 id="非等值连接"><a href="#非等值连接" class="headerlink" title="非等值连接"></a>非等值连接</h3><p>查询员工的工资和工资级别</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> salary<span class="token punctuation">,</span>grade_level
<span class="token keyword">from</span> employees e<span class="token punctuation">,</span>job_grades g
<span class="token keyword">where</span> salary <span class="token operator">BETWEEN</span> g<span class="token punctuation">.</span>lowest_sal <span class="token operator">and</span> g<span class="token punctuation">.</span>highest_sal<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>



<h3 id="自连接"><a href="#自连接" class="headerlink" title="自连接"></a>自连接</h3><p>查询员工名和上级的名称</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> e<span class="token punctuation">.</span>employee_id<span class="token punctuation">,</span>e<span class="token punctuation">.</span>last_name<span class="token punctuation">,</span>m<span class="token punctuation">.</span>employee_id<span class="token punctuation">,</span>m<span class="token punctuation">.</span>last_name
<span class="token keyword">from</span> employees e<span class="token punctuation">,</span>employees m
<span class="token keyword">where</span> e<span class="token punctuation">.</span>manager_id <span class="token operator">=</span> m<span class="token punctuation">.</span>employee_id<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>



<h2 id="二、sql199"><a href="#二、sql199" class="headerlink" title="二、sql199"></a>二、sql199</h2><p>语法：</p>
<p>​            select 查询列表</p>
<p>​            from 表1 别名 【连接类型】</p>
<p>​            join 表2 别名 on 连接条件</p>
<p>​            【where 筛选条件】</p>
<p>​            【group by 分组】</p>
<p>分类：</p>
<p>内连接：inner</p>
<p>外连接</p>
<p>​            左外 left 【outer】</p>
<p>​            右外 right 【outer】</p>
<p>​            全外 full 【outer】</p>
<p>交叉连接 cross</p>
<h3 id="一）内连接"><a href="#一）内连接" class="headerlink" title="一）内连接"></a>一）内连接</h3><p>语法：</p>
<p>select 查询列表</p>
<p>from 表1 别名</p>
<p>inner join 表2 别名</p>
<p>on 连接条件</p>
<p>分类：</p>
<p>等值、非等值、自连接</p>
<p>特点：</p>
<p>添加排序、分组、筛选</p>
<p>inner可以省略</p>
<p>筛选条件放在where后面，连接条件放在on后面，提高分离性，便于阅读</p>
<p>inner join连接和sql192语法中的等值连接效果是一样的</p>
<h4 id="1）等值连接"><a href="#1）等值连接" class="headerlink" title="1）等值连接"></a>1）等值连接</h4><p>查询员工名、部门名</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> last_name<span class="token punctuation">,</span>department_name
<span class="token keyword">from</span> employees e
<span class="token keyword">INNER</span> <span class="token keyword">JOIN</span> departments d
<span class="token keyword">on</span> e<span class="token punctuation">.</span>department_id <span class="token operator">=</span> d<span class="token punctuation">.</span>department_id<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>查询名字中包含e的员工名和工种名（添加筛选）</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">SELECT</span> last_name<span class="token punctuation">,</span>job_title
<span class="token keyword">from</span> employees e
<span class="token keyword">INNER</span> <span class="token keyword">JOIN</span> jobs j
<span class="token keyword">on</span> e<span class="token punctuation">.</span>job_id <span class="token operator">=</span> j<span class="token punctuation">.</span>job_id
<span class="token keyword">WHERE</span> e<span class="token punctuation">.</span>last_name <span class="token operator">like</span> <span class="token string">'%e%'</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>查询部门个数&gt;3的城市名和部门个数，（添加分组+筛选）</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">SELECT</span> city<span class="token punctuation">,</span><span class="token function">count</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span> 个数
<span class="token keyword">from</span> departments d
<span class="token keyword">INNER</span> <span class="token keyword">JOIN</span> locations l
<span class="token keyword">on</span> d<span class="token punctuation">.</span>location_id <span class="token operator">=</span> l<span class="token punctuation">.</span>location_id
<span class="token keyword">GROUP</span> <span class="token keyword">BY</span> city
<span class="token keyword">HAVING</span> <span class="token function">COUNT</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span><span class="token operator">&gt;</span><span class="token number">3</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>查询哪个部门的员工个数&gt;3的部门名和员工个数，并按个数降序（添加排序）</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">SELECT</span> department_name<span class="token punctuation">,</span><span class="token function">count</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span> 个数
<span class="token keyword">from</span> employees e
<span class="token keyword">INNER</span> <span class="token keyword">JOIN</span> departments d
<span class="token keyword">on</span> e<span class="token punctuation">.</span>department_id <span class="token operator">=</span> d<span class="token punctuation">.</span>department_id
<span class="token keyword">GROUP</span> <span class="token keyword">BY</span> department_name
<span class="token keyword">HAVING</span> <span class="token function">COUNT</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span><span class="token operator">&gt;</span><span class="token number">3</span>
<span class="token keyword">ORDER</span> <span class="token keyword">BY</span> <span class="token function">count</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span> <span class="token keyword">desc</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>查询员工名、部门名、工种名，并按部门名降序</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">SELECT</span> last_name<span class="token punctuation">,</span>department_name<span class="token punctuation">,</span>job_title
<span class="token keyword">from</span> employees e
<span class="token keyword">INNER</span> <span class="token keyword">JOIN</span> departments d <span class="token keyword">on</span> e<span class="token punctuation">.</span>department_id <span class="token operator">=</span> d<span class="token punctuation">.</span>department_id
<span class="token keyword">INNER</span> <span class="token keyword">JOIN</span> jobs j <span class="token keyword">on</span>  j<span class="token punctuation">.</span>job_id <span class="token operator">=</span> e<span class="token punctuation">.</span>job_id
<span class="token keyword">ORDER</span> <span class="token keyword">BY</span> department_name <span class="token keyword">desc</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h4 id="2）非等值连接"><a href="#2）非等值连接" class="headerlink" title="2）非等值连接"></a>2）非等值连接</h4><p>查询员工的工资级别</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> salary<span class="token punctuation">,</span>grade_level
<span class="token keyword">from</span> employees e
<span class="token keyword">INNER</span> <span class="token keyword">JOIN</span> job_grades g
<span class="token keyword">on</span> e<span class="token punctuation">.</span>salary <span class="token operator">BETWEEN</span> g<span class="token punctuation">.</span>lowest_sal <span class="token operator">and</span> g<span class="token punctuation">.</span>highest_sal<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>查询工资级别的个数&gt;20的个数，并且按工资级别降序</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> <span class="token function">count</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span><span class="token punctuation">,</span>salary<span class="token punctuation">,</span>grade_level
<span class="token keyword">from</span> employees e
<span class="token keyword">INNER</span> <span class="token keyword">JOIN</span> job_grades g
<span class="token keyword">on</span> e<span class="token punctuation">.</span>salary <span class="token operator">BETWEEN</span> g<span class="token punctuation">.</span>lowest_sal <span class="token operator">and</span> g<span class="token punctuation">.</span>highest_sal
<span class="token keyword">GROUP</span> <span class="token keyword">BY</span> grade_level
<span class="token keyword">HAVING</span> <span class="token function">count</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span><span class="token operator">&gt;</span> <span class="token number">20</span>
<span class="token keyword">ORDER</span> <span class="token keyword">BY</span> grade_level <span class="token keyword">desc</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h4 id="3）自连接"><a href="#3）自连接" class="headerlink" title="3）自连接"></a>3）自连接</h4><p>查询姓名中包含字符k的员工的名字、上级的名字</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> e<span class="token punctuation">.</span>last_name<span class="token punctuation">,</span>m<span class="token punctuation">.</span>last_name
<span class="token keyword">from</span> employees e
<span class="token keyword">join</span> employees m
<span class="token keyword">on</span> e<span class="token punctuation">.</span>manager_id <span class="token operator">=</span> m<span class="token punctuation">.</span>employee_id
<span class="token keyword">where</span> e<span class="token punctuation">.</span>last_name <span class="token operator">like</span> <span class="token string">'%k%'</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="二、外连接"><a href="#二、外连接" class="headerlink" title="二、外连接"></a>二、外连接</h3><p>应用场景：用于查询一个表中有，另一个表中没有的记录</p>
<p>特点：</p>
<p>1、外连接的查询结果为主表中的所有记录</p>
<p>​        如果从表中有和它匹配的，则显示匹配的值</p>
<p>​        如果从表中没有和它匹配的，则显示null</p>
<p>​        外连接查询结果=内连接结果+主表中有而从表没有的记录</p>
<p>2、左外连接：left join左边的是主表</p>
<p>​      右外连接：right join右边的是主表</p>
<p>3、左外和右外交换两个表的顺序、可以实现相同的效果</p>
<p>4、全外连接=左外+右外的并集</p>
<p>​                    =内连接+表1有表2没有+表1没有表2有</p>
<p>查询男朋友 不在男神表的女神名</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> b<span class="token punctuation">.</span>name
<span class="token keyword">from</span> beauty b
<span class="token keyword">LEFT</span> <span class="token keyword">OUTER</span> <span class="token keyword">JOIN</span> boys bo
<span class="token comment">#右外连接：from boys bo right outer join beauty b</span>
<span class="token keyword">on</span> b<span class="token punctuation">.</span>boyfriend_id <span class="token operator">=</span> bo<span class="token punctuation">.</span>id
<span class="token keyword">where</span> bo<span class="token punctuation">.</span>id <span class="token operator">is</span> <span class="token boolean">null</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>查询哪个部门没有员工</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> department_name
<span class="token keyword">from</span> departments d
<span class="token keyword">LEFT</span> <span class="token keyword">JOIN</span> employees e
<span class="token keyword">on</span> e<span class="token punctuation">.</span>department_id <span class="token operator">=</span> d<span class="token punctuation">.</span>department_id
<span class="token keyword">WHERE</span> e<span class="token punctuation">.</span>employee_id <span class="token operator">is</span> <span class="token boolean">null</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="全外连接"><a href="#全外连接" class="headerlink" title="全外连接"></a>全外连接</h3><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">SELECT</span> b<span class="token punctuation">.</span><span class="token operator">*</span><span class="token punctuation">,</span>bo<span class="token punctuation">.</span><span class="token operator">*</span>
<span class="token keyword">FROM</span> beauty b
<span class="token keyword">FULL</span> <span class="token keyword">OUTER</span> <span class="token keyword">JOIN</span> boys bo
<span class="token keyword">ON</span> b<span class="token punctuation">.</span><span class="token identifier"><span class="token punctuation">`</span>boyfriend_id<span class="token punctuation">`</span></span> <span class="token operator">=</span> bo<span class="token punctuation">.</span>id<span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>mysql里使用union模拟实现</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> b<span class="token punctuation">.</span><span class="token operator">*</span><span class="token punctuation">,</span>bo<span class="token punctuation">.</span><span class="token operator">*</span>
<span class="token keyword">from</span> beauty b
<span class="token keyword">LEFT</span> <span class="token keyword">OUTER</span> <span class="token keyword">JOIN</span> boys bo
<span class="token keyword">on</span> b<span class="token punctuation">.</span>boyfriend_id <span class="token operator">=</span> bo<span class="token punctuation">.</span>id
<span class="token keyword">union</span>
<span class="token keyword">select</span> b<span class="token punctuation">.</span><span class="token operator">*</span><span class="token punctuation">,</span>bo<span class="token punctuation">.</span><span class="token operator">*</span>
<span class="token keyword">from</span> beauty b
<span class="token keyword">RIGHT</span> <span class="token keyword">JOIN</span> boys bo
<span class="token keyword">on</span> b<span class="token punctuation">.</span>boyfriend_id <span class="token operator">=</span> bo<span class="token punctuation">.</span>id<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="交叉连接"><a href="#交叉连接" class="headerlink" title="交叉连接"></a>交叉连接</h3><p>笛卡尔乘积</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> b<span class="token punctuation">.</span><span class="token operator">*</span><span class="token punctuation">,</span>bo<span class="token punctuation">.</span><span class="token operator">*</span>
<span class="token keyword">from</span> beauty b
<span class="token keyword">CROSS</span> <span class="token keyword">join</span> boys bo<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>


                
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